Chapter 6 Chapter 6

Arrays, Pointers, and StringsArrays, Pointers, and Strings

2008-11-26

MemoryMemory

Compiler

User programint money=50;

變數 a ，在某個箱子 ( 位址 ) ，放 50 元變數對應表

MemoryMemory

2650x0045FF00

520x0045FF04

620x00450008

780x0045000B

…………………………………………………………………………

…………………………………………………………………………

…………………………………………………………………………

0x00000000

0xFFFFFFFF

PointerPointer

► #include <stdio.h>#include <stdio.h>

► int main(void)int main(void)► {{► int a=100;int a=100;► printf(“a=%d",a);printf(“a=%d",a);► printf(“,location of a=%p",&a); printf(“,location of a=%p",&a); ► }}

Output: a=100,location of a=0023FF74

PointerPointer

► #include <stdio.h>#include <stdio.h>

► int main(void)int main(void)► {{► int *p = 100;int *p = 100;► printf(“*p=%d”,*p); //printf(“*p=%d”,*p); // 錯誤、程式當機 錯誤、程式當機 ► }}

Output: ErrorMemory

PointerPointer

► #include <stdio.h>#include <stdio.h>

► int main(void)int main(void)► {{► int a=100;int a=100;► int *p=&a; int *p=&a; ► printf(“\n a = %d”,a); // Value aprintf(“\n a = %d”,a); // Value a► printf(“\n &a = %p”,&a); // Location aprintf(“\n &a = %p”,&a); // Location a► printf(“\n *p = %d",*p); // Value aprintf(“\n *p = %d",*p); // Value a► printf(“\n &p = %p”,&p); // Location pprintf(“\n &p = %p”,&p); // Location p► printf(“\n p = %p”,p); // Location aprintf(“\n p = %p”,p); // Location a► }}

Output:

a = 100&a = 0023FF74*p = 100&p = 0023FF70p = 0023FF74

► #include <stdio.h>#include <stdio.h>► int main(void)int main(void)► {{► int a=100;int a=100;► int b=200;int b=200;► int *p = &a;int *p = &a; ► ► printf("\n a = %d",a); // Value aprintf("\n a = %d",a); // Value a► printf("\n &a = %p",&a); // Location a printf("\n &a = %p",&a); // Location a ► printf("\n b = %d",b); // Value b printf("\n b = %d",b); // Value b ► printf("\n &b = %p",&b); // Location b printf("\n &b = %p",&b); // Location b ► printf("\n *p = %d",*p); // Value aprintf("\n *p = %d",*p); // Value a► printf("\n &p = %p",&p); // Location pprintf("\n &p = %p",&p); // Location p► printf("\n p = %p",p); // Location aprintf("\n p = %p",p); // Location a► printf("\n");printf("\n");

► *p = &b;*p = &b;► printf("\n a = %p",a); // Value a is modifiedprintf("\n a = %p",a); // Value a is modified► printf("\n &a = %p",&a); // Location a printf("\n &a = %p",&a); // Location a ► printf("\n b = %d",b); // Value b printf("\n b = %d",b); // Value b ► printf("\n &b = %p",&b); // Location b printf("\n &b = %p",&b); // Location b ► printf("\n *p = %p",*p); // Location bprintf("\n *p = %p",*p); // Location b► printf("\n &p = %p",&p); // Location pprintf("\n &p = %p",&p); // Location p► printf("\n p = %p",p); // Location aprintf("\n p = %p",p); // Location a► }}

a = 100&a = 0023FF74b = 200&b = 0023FF70*p = 100&p = 0023FF6Cp = 0023FF74

a = 0023FF70&a = 0023FF74b = 200&b = 0023FF70*p = 0023FF70&p = 0023FF6Cp = 0023FF74

PointerPointer

► int i=3,j=5, *p=&i, *q=&j, *r;int i=3,j=5, *p=&i, *q=&j, *r;► double x;double x;

p==&ip==&i p==(&i)p==(&i) 11

**&p**&p *(*(&p))*(*(&p)) 33

r=&xr=&x r=(&x)r=(&x) /*illegal*//*illegal*/

7**p / *q+77**p / *q+7 (((7*(*p)))/(*q))+7(((7*(*p)))/(*q))+7 1111

ExampleExample

► void swap(int *p, int *q)void swap(int *p, int *q)► {{► int tmp;int tmp;► tmp = *p;tmp = *p;► *p = *q;*p = *q;► *q = tmp;*q = tmp;► }}

► #include <stdio.h>#include <stdio.h>► void swap(int *, int *);void swap(int *, int *);

► int main (void)int main (void)► {{► int i=4,j=5;int i=4,j=5;► swap(&i,&j);swap(&i,&j);► printf(“i=%d,j=%d”,i,j);printf(“i=%d,j=%d”,i,j);► return 0;return 0;► }}

3i

5j

*p *q

tmp

Output: i=5,j=4

Array addressArray address

► int i,my_array[10];int i,my_array[10];► for (i=0;i<10;i++)for (i=0;i<10;i++)► my_array[i] = i;my_array[i] = i;

► printf(“%p”,my_array); printf(“%p”,my_array); ► printf(“%d”,my_array[0]);printf(“%d”,my_array[0]);► printf(“%p”,&my_array[0]);printf(“%p”,&my_array[0]);► printf(“%d”,my_array[2]);printf(“%d”,my_array[2]);► printf(“%p”,&my_array[2]);printf(“%p”,&my_array[2]);

0023FF3000023FF3020023FF38

► #include <stdio.h>#include <stdio.h>

► int fun(int *p)int fun(int *p)► {{► *p = 70;*p = 70;► }}

► int main(void)int main(void)► {{► int i,my_array[10],sum;int i,my_array[10],sum;► for (i=0;i<10;i++)for (i=0;i<10;i++)► my_array[i] = i;my_array[i] = i;

► fun_sum(my_array);fun_sum(my_array);► fun_sum(&my_array[0]);fun_sum(&my_array[0]);► fun_sum(&my_array[5]);fun_sum(&my_array[5]);► ► }}

n = 7 20 8030 5040 70 90

0 32 51 4 6

pass 1:

n = 7 20 7040 8030 50 90

0 32 51 4 6

pass 2:

n = 7 20 5040 8030 70 90

0 32 51 4 6

pass 3:

n = 7 20 3040 7090 80 50

0 32 51 4 6

n = 7 20 5040 8030 70 90

0 32 51 4 6

pass 4:

► #include <stdio.h>#include <stdio.h>► void swap(int *p, int *q)void swap(int *p, int *q)► {{► int tmp;int tmp;► tmp = *p;tmp = *p;► *p = *q;*p = *q;► *q = tmp;*q = tmp;► }}► void bubble(int a[], int n)void bubble(int a[], int n)► {{► int i,j;int i,j;► for (i=0;i<n-1;i++)for (i=0;i<n-1;i++)► {{► for (j=0;j<(n-i);j++) for (j=0;j<(n-i);j++) ► { { ► if (a[j] > a[j+1])if (a[j] > a[j+1])► swap(&a[j], &a[j+1]);swap(&a[j], &a[j+1]);► }}► }}► }}► void main()void main()► {{► int a[] = {7, 3, 66, 35, -5, 22, -77, 2};int a[] = {7, 3, 66, 35, -5, 22, -77, 2};► int i;int i;► bubble(a,8); bubble(a,8); ► for(i=0;i<8;i++)for(i=0;i<8;i++)► printf("%d ",a[i]);printf("%d ",a[i]);► }}

7 3 66 35 -5 22 -77 2

n=8

void bubble(int a[], int n)void bubble(int a[], int n){{ int i,j;int i,j; for (i=0;i<n-1;i++)for (i=0;i<n-1;i++) {{ for (j=0;j<(n-i);j++) for (j=0;j<(n-i);j++) { { if (a[j] > a[j+1])if (a[j] > a[j+1]) swap(&a[j], &a[j+1]);swap(&a[j], &a[j+1]); }} }}}} Page:258

i=0 j=0

Time complexity of bubble sortTime complexity of bubble sort

► A bubble sort is very inefficient. If the size of the A bubble sort is very inefficient. If the size of the array is array is nn, then the number of comparisons , then the number of comparisons performed is proportional to performed is proportional to nn22. .

6.8 Dynamic Memory Allocation With 6.8 Dynamic Memory Allocation With calloc()calloc() and and malloc()malloc()

► C provides the functions C provides the functions calloc()calloc() and and malloc()malloc() in the standard in the standard library, and their function prototypes are in stdlib.h. library, and their function prototypes are in stdlib.h.

► The name calloc stands for The name calloc stands for “contiguous allocation,”“contiguous allocation,” and the and the name malloc stands for name malloc stands for “memory allocation.”“memory allocation.”

► The programmer uses The programmer uses calloc()calloc() and and malloc()malloc() to dynamically to dynamically create space for arrays, structures(Chap.9), and create space for arrays, structures(Chap.9), and unions(Chap.10). unions(Chap.10).

I can not predict the requirement of user memory

calloccalloc

► int *a = int *a = calloc(5,sizeof(int));calloc(5,sizeof(int));

► free(a);free(a);

a

*a*a

要 5 塊 int 大小的記憶體空間

特別注意

mallocmalloc

► int *a = int *a = malloc(5*sizeof(int));malloc(5*sizeof(int));

► free(a);free(a);

a

*a*a

要 5 塊 int 大小的記憶體空間

特別注意

calloc and malloccalloc and malloc

► calloccalloc is similar to malloc, but the main difference is similar to malloc, but the main difference is that the values stored in the allocated memory is that the values stored in the allocated memory space is space is zerozero by default. With malloc, the allocated by default. With malloc, the allocated memory could have any value. memory could have any value.

► calloccalloc requires requires two argumentstwo arguments. The first is the . The first is the number of variables you'd like to allocate memory number of variables you'd like to allocate memory for. The second is the size of each variable. for. The second is the size of each variable.

Example 1Example 1► #include <stdio.h>#include <stdio.h>► #include <stdlib.h>#include <stdlib.h>

► int main(void)int main(void)► {{► int *a;int *a;► int n=4;int n=4;► ► a = a = calloc(n,sizeof(int));calloc(n,sizeof(int));► ► *a = 10;*a = 10;► *(a+1) = 20;*(a+1) = 20;► ► printf("%d %d",*a,*(a+1));printf("%d %d",*a,*(a+1));

► free(a);free(a);► }}

10

*a*a *(a+1)*(a+1)

20

*(a+2)*(a+2) *(a+3)*(a+3)

Output: 10 20Output: 10 20

► #include <stdio.h>#include <stdio.h>► #include <stdlib.h>#include <stdlib.h>

► int main(void)int main(void)► {{► int *a;int *a;► int n=10;int n=10;► int i;int i;► int sum=0;int sum=0;► a = a = calloc(n,sizeof(int));calloc(n,sizeof(int));► ► for (i=0;i<n;i++)for (i=0;i<n;i++)► {{► *(a+i) = 10;*(a+i) = 10;► sum += *(a+i);sum += *(a+i);► }}► printf("%d",sum);printf("%d",sum);► }}

Example 2 Example 2

Output: 100Output: 100

Example 3Example 3► #include <stdio.h>#include <stdio.h>► #include <stdlib.h>#include <stdlib.h>

► void fun(int *a)void fun(int *a)► {{► *a = 30; *a = 30; ► *(a+1) = 40;*(a+1) = 40;► }}

► void main()void main()► {{► int *a = calloc(5,sizeof(int));int *a = calloc(5,sizeof(int));► *a = 10;*a = 10;► *(a+1) = 20;*(a+1) = 20;► ► printf("\n*a=%d,*(a+1)=%d",*a,*(a+1));printf("\n*a=%d,*(a+1)=%d",*a,*(a+1));► fun(&a[2]); fun(&a[2]); ► printf("\n*a=%d,*(a+1)=%d,*(a+2)=%d,*(a+3)=printf("\n*a=%d,*(a+1)=%d,*(a+2)=%d,*(a+3)=

%d",*a,*(a+1),*(a+2),*(a+3));%d",*a,*(a+1),*(a+2),*(a+3));► free(a);free(a);► }}

10

*a*a *(a+1)*(a+1)

20

*(a+2)*(a+2) *(a+3)*(a+3)

Output:Output:

*(a+3)*(a+3)

*a=10,*(a+1)=20*a=10,*(a+1)=20,*(a+2)=30,*(a+3)=40

Example 4Example 4► #include <stdio.h>#include <stdio.h>► #include <stdlib.h> #include <stdlib.h>

► int main() {int main() {► float *calloc1,*malloc1 ;float *calloc1,*malloc1 ;► int i;int i;

► calloc1 = calloc(3, sizeof(float));calloc1 = calloc(3, sizeof(float));► malloc1 = malloc(3 * sizeof(float));malloc1 = malloc(3 * sizeof(float));

► if(calloc1!=NULL && malloc1!=NULL!=NULL) {if(calloc1!=NULL && malloc1!=NULL!=NULL) {► for(i=0 ; i<3 ; i++) {for(i=0 ; i<3 ; i++) {► printf("calloc1[%d] holds %.0f, ", i, calloc1[i]);printf("calloc1[%d] holds %.0f, ", i, calloc1[i]);► printf("malloc1[%d] holds %.0f\n", i, malloc1[i]);printf("malloc1[%d] holds %.0f\n", i, malloc1[i]);► }}► free(calloc1);free(calloc1);► free(malloc1); free(malloc1); ► }}► else {else {► printf("Not enough memory\n"); printf("Not enough memory\n"); ► }}► }}

Example 5Example 5

► Use Use scanfscanf to get to get nn► Use Use calloccalloc to get the to get the nn memory space memory space► Use Use *a*a to initial the value (0,1,2,3,4,5…..) to initial the value (0,1,2,3,4,5…..)► Use Use for-loopfor-loop to get the sum to get the sum

► #include <stdio.h>#include <stdio.h>► #include <stdlib.h>#include <stdlib.h>

► int main(void)int main(void)► {{► int *a; int *a; ► int i,n,sum=0; int i,n,sum=0; ► while(n!=999)while(n!=999)► {{► printf("\n\nInput n (Esc:999) --> ");printf("\n\nInput n (Esc:999) --> ");► scanf("%d",&n);scanf("%d",&n);► a = calloc(n,sizeof(int));a = calloc(n,sizeof(int));► for (i=0;i<n;i++)for (i=0;i<n;i++)► {{► *(a+i) = i; *(a+i) = i; ► sum += *(a+i);sum += *(a+i);► }}► printf("n=%d,sum=%d",n,sum); printf("n=%d,sum=%d",n,sum); ► sum=0;sum=0;► free(a);free(a);► } } ► }}

Example 6Example 6

► Input the array size Input the array size nn, and do the following tasks., and do the following tasks.

► 1 create space for an array of size n1 create space for an array of size n► 2 fill the array with randomly distributed digits2 fill the array with randomly distributed digits► 3 print the array and the sum of its element3 print the array and the sum of its element► 4 release the space4 release the space

► #include <stdio.h>#include <stdio.h>► #include <stdlib.h>#include <stdlib.h>► #include <time.h>#include <time.h>

► void fill_array(int *a, int n);void fill_array(int *a, int n);► int sum_array(int *a, int n);int sum_array(int *a, int n);► void wrt_array(int *a, int n);void wrt_array(int *a, int n);

► void fill_array(int *a, int n)void fill_array(int *a, int n)► {{► int i;int i;► for (i=0;i<n;++i)for (i=0;i<n;++i)► a[i] = rand() % 19 – 9;a[i] = rand() % 19 – 9;► }}

► int sum_array(int *a, int n)int sum_array(int *a, int n)► {{► int i, sum=0;int i, sum=0;► for (i=0;i<n;++i)for (i=0;i<n;++i)► sum += a[i];sum += a[i];► return sum;return sum;► }}

► void wrt_array(int *a, int n)void wrt_array(int *a, int n)► {{► int i;int i;► printf(“a= [“);printf(“a= [“);► for (i=0;i<n;++i)for (i=0;i<n;++i)► printf(“%d%s”,a[i],((i<n-1) ? “,” : “]\n”));printf(“%d%s”,a[i],((i<n-1) ? “,” : “]\n”));► }}

► int main(void)int main(void)► {{► int *a,n;int *a,n;► srand(time(NULL)); /*seed the random number generator*/srand(time(NULL)); /*seed the random number generator*/► printf(“\n%s\n”,printf(“\n%s\n”,► “ “This program does the following repeatedly:\n”This program does the following repeatedly:\n”► “ “\n”\n”► “ “ 1 create space for an array of size n\n”1 create space for an array of size n\n”► “ “ 2 fill the array with randomly distributed digits\n”2 fill the array with randomly distributed digits\n”► “ “ 3 print the array and the sum of its element\n”3 print the array and the sum of its element\n”► “ “ 4 release the space\n”);4 release the space\n”);

► for ( ; ;)for ( ; ;)► {{► printf(“Input n: “);printf(“Input n: “);► if (scanf(“%d”,&n) != 1 || n< 1)if (scanf(“%d”,&n) != 1 || n< 1)► break;break;► putchar(‘\n’);putchar(‘\n’);► a = calloc(n,sizeof(int));a = calloc(n,sizeof(int));► fill_array(a,n);fill_array(a,n);► wrt_array(a,n);wrt_array(a,n);► printf(“sum=%d\n”,sum_array(a,n));printf(“sum=%d\n”,sum_array(a,n));► free(a);free(a);► }}► printf(“\nBYE-BYE!!”);printf(“\nBYE-BYE!!”);► return 0;return 0;► }}

Offsetting the PointerOffsetting the Pointer

► For arrays (vectors) intended for mathematical use, For arrays (vectors) intended for mathematical use, we often want to index the arrays from 1 instead 0. we often want to index the arrays from 1 instead 0.

► We can do something like the following:We can do something like the following:

int n;int n; double *a;double *a; …… a = calloc(n+1, sizeof(double));a = calloc(n+1, sizeof(double));

► We can disregard a[0] and use a[1],…,a[n] as We can disregard a[0] and use a[1],…,a[n] as needed.needed.